The technique of colorimetric chemical analysis of liquids is well established. The classic method involves transmitting a light beam transversely through a transparent cell (for example, a glass cylinder) containing a liquid sample to be tested. To the liquid sample (such as water containing a foreign substance whose concentration is to be measured) are added appropriate types and amounts of reagents which will react with the foreign substance in the sample and form a color. The degree of transmittance of the light beam through the cell containing the liquid sample will be directly affected by the degree of color formation in the liquid sample (resulting from the concentration of the foreign substance in the liquid sample). The degree of light transmittance through the liquid sample, as measured by a photodetector, is compared with other samples containing known concentrations of the same foreign substance in order to determine the quantitative amount of the foreign substance in the tested sample. When using this classic technique a control sample of liquid (which does not contain any of the color forming reagents) in a separate cell is used in the colorimeter as a baseline for light transmittance.
One of the main disadvantages of the classic technique is that separate sample cells are used for the sample to be tested, the control sample (i.e., without reagents), and the samples of known concentrations of the foreign substance. Because of the variance in the amount of light absorbance by the walls of the different sample cells, there are inherent inaccuracies in the results obtained from the testing.
As is known in the art, the concentration of the foreign substance in the liquid sample is related to the photocell current developed by the photocell, the concentration being proportional to log (I ref./I sig.), where I ref. is the photocell current when no color-forming reagent is in the sample and I sig. is the photocell current after the color in the sample has been developed.
The simplest type of analyzer would have one sample cell, one light beam, and one photodetector. The instrument would require periodic standardization by introducing a pure liquid sample (i.e., a liquid sample not containing the foreign substance being tested for) which contains no color-producing means, and then making appropriate adjustments so as to make current A (the photocell output) equal to current B (a fixed current) such that the log amp output is zero. Span adjustment is achieved by introducing a standard solution of known concentration of foreign substance and adjusting the log amp gain until the instrument reads the proper value. The instrument would now read the proper value. Such an instrument has no provision for automatically compensating for drifts due to sample aging, temperature, or dirty optical windows; hence, it is suitable only for less exacting analysis.
Some of the disadvantages associated with use of the classic technique have been alleviated by means of a technique involving a single simple cell and a beam splitter which directs a portion of the light beam through a filter to a reference detector and the main part of the light beam through a second filter to a sample detector. The wavelengths of light passed by the two filters are predetermined based upon the particular sample parameter being tested. However, this technique also has associated disadvantages. For example, it requires that two photodetectors be used. The two photodetectors may drift or vary with temperature at different rates, particularly since they are measuring different wavelengths of light. Also, if there is turbidity in the sample it could absorb light differently at the different wavelengths. Another disadvantage is that it may not be possible to find a reference wavelength which is not absorbed to some extent by the color in the sample cell.
Another known technique involves an analyzer in which a single light source is used in conjunction with two sample cells, two color filters of the same type, and two photodetectors. The liquid sample is first introduced into sample cell B without any color-forming reagents present. The light transmitted through the sample reaches photodetector B (where current B is generated). The liquid sample is then transferred to sample cell A (with color forming reagents added enroute). The light transmitted through the prepared sample reaches photodetector A (where current A is generated). Such an analyzer is not affected by lamp aging or temperature-related drifts in the sample but it does have the disadvantage of having two sets of optical surfaces which may become dirty at different rates. Also, the two photodetectors may have different temperature drift characteristics.
Yet another technique involves a dual beam photometer. A light beam from a single light source is divided into two separate beams, one of the beams being directed through a sample cell containing the reference standard and the other beam being directed through a separate sample cell containing the sample to be tested. The same photodetector is used to measure light transmittance through both the reference cell and the sample cell. There are inherent limitations associated with this technique, however. For example, the sample cell and the reference cell may be of slightly different sizes. Also, the surface conditions of the reference cell and sample cell may be different. These types of differences, of course, may have a significant effect on the accuracy of the technique.